Spin flip
A black hole spin-flip occurs when the spin axis of a rotating black hole undergoes a sudden change in orientation due to absorption of a second (smaller) black hole. Spin-flips are believed to be a consequence of galaxy mergers, when two supermassive black holes form a bound pair at the center of the merged galaxy and coalesce after emitting gravitational waves. Spin-flips are significant astrophysically since a number of physical processes are associated with black hole spins; for instance, jets in active galaxies are believed to be launched parallel to the spin axes of supermassive black holes. Black hole spin-flips were first described by David Merritt and Ron Ekers, who also proposed that a particular class of radio galaxy, the X-shaped radio galaxies, could be explained in terms of spin-flips.D. Merritt and R. Ekers (2002), "Tracing black hole mergers through radio lobe morphology", Science 297, 1310. The first, fully self-consistent computer simulations to produce spin-flips were carried out by Manuela Campanelli and collaborators at the Center for Computational Relativity and Gravitation in 2007. M. Campanelli et al. (2007), "Spin flips and precession in black-hole-binary mergers", Phys. Rev. D. 75, 064030. Physics of Spin-Flips A spin-flip is a late stage in the evolution of a binary black hole. The binary consists of two black holes, with masses M_1 and M_2 , that revolve around their common center of mass. The total angular momentum J of the binary system is the sum of the angular momentum of the orbit, {L} , plus the spin angular momenta {S}_{1,2} of the two holes: \mathbf{J}_{\rm init} = \mathbf{L}_{\rm orb} + \mathbf{S}_1 + \mathbf{S}_2. If the orbital separation is sufficiently small, emission of energy and angular momentum in the form of gravitational radiation will cause the orbital separation to drop. Eventually, the smaller hole M_2 reaches the innermost stable circular orbit, or ISCO, around the larger hole. Once the ISCO is reached, there no longer exists a stable orbit, and the smaller hole plunges into the larger hole, coalescing with it. The final angular momentum after coalescence is just \mathbf{J}_{\rm final} = \mathbf{S}, the spin angular momentum of the single, coalesced hole. Neglecting the angular momentum that is carried away by gravitational waves during the final plunge -- which is smallBaker, J. et al. (2006), "Gravitational wave extraction from an inspiraling configuration of merging black holes", Phys. Rev. Lett. 96, 11102. -- conservation of angular momentum implies \mathbf{S} \approx \mathbf{L}_{\rm ISCO} + \mathbf{S}_1 + \mathbf{S}_2. S_2 is of order (M_2/M_1)^2 times S_1 and can be ignored if M_2 is much smaller than M_1 . Making this approximation, \mathbf{S} \approx \mathbf{L}_{\rm ISCO} + \mathbf{S}_1. This equation states that the final spin of the hole is the sum of the larger hole's initial spin plus the orbital angular momentum of the smaller hole at the last stable orbit. Since the vectors S_1 and L are generically oriented in different directions, S will point in a different direction than S_1 -- a spin-flip. The angle by which the black hole's spin re-orients itself depends on the relative size of L_{\rm ISCO} and S_1 , and on the angle between them. At one extreme, if S_1 is very small, the final spin will be dominated by L_{\rm ICSO} and the flip angle can be large. At the other extreme, suppose that the larger black hole is a maximally-rotating Kerr black hole initially. Its spin angular momentum is then of order S_1 \approx GM_1^2/c. The orbital angular momentum of the smaller hole at the ISCO depends on the direction of its orbit, but is of order L_{\rm ICSO} \approx GM_1M_2/c. Comparing these two expressions, it follows that even a fairly small hole, with mass about one-fifth that of the larger hole, can reorient the larger hole by 90 degrees or more. See also # Supermassive black hole # Rotational Brownian motion # Gravitational waves # Radio galaxy References External links * Massive black hole binary evolution An article on binary black holes. * Scientists Detect "Smoking Gun" of Colliding Black Holes Category:Radio galaxies Category:Supermassive black holes Category:Black holes Category:Nuclear fusion